I think Qt is quite awesome because it can run on Arm architectures and is packed with features.

To answer your question, what do I need to use? At the moment, the only reason I want to compile Qt is to use only 1 class - QMediaPlayer which belongs to the multimedia package. So I definitely do not need to compile the whole thing, but I couldn't find a guide that shows me how to compile it separately.

Then you can follow the guide but call configure directly in qtbase, this way you will only have this module. Next step, build and install it.

From thee, create a new subfolder for qtmultimedia go into it and call your installed qmake with /path/to/qtmultimedia/sources as parameter. Then make and sudo make install. Take the time to look at the configuration tests to see whether it finds everything it needs to build the module